This invention relates to separation processes and equipment, and more particularly to mixer settlers, such as those used in solvent-extraction processes during minerals processing.
Mixer settlers generally comprise two stages. A first stage comprises mixing two immiscible fluids (e.g., an organic phase and an aqueous phase) in one or more mixers to create an dispersion which facilitates mass transfer of a target mineral from one of the phases to the other. A second quiescent settling stage allows the two phases to separate from their suspended state—according to their density. Typically, the second stage utilizes a settling tank largely resembling a pool. A fixed organic overflow weir extends across the entire width of the settling tank and serves to “skim” the very top of the lighter organic phase. An adjustable aqueous weir also extends across the entire width of the settling tank to collect the heavier aqueous phase. The adjustable aqueous weir also enables control of operating level within the settler. A shortcoming of such mixer settler designs is that because the width of the settler and, therefore, the length of the adjustable aqueous weir along the width of the settling tank in part, determines the total capacity of the mixer settler, adjustable aqueous weirs have become increasingly large and complex. Large adjustable weirs are cumbersome and demonstrate problems when adjusting their level along the width of the settling tank. Moreover, adjusting the positional height of the adjustable weir must be performed at several discrete locations (i.e., “points of adjustment”) along the width of the settling tank (refer to FIG. 13). Therefore, simple, fast, fine-tuned adjustments during a separation process are often difficult. Additionally, “overflow”, which passes over conventional organic and aqueous weirs is very turbulent and can readily entrain air and promote formation of undesirable impurities in the system.
FIGS. 12 and 13 illustrate respective side and top views of one example of a conventional mixer settler. A primary mixer and one or more auxiliary mixers are provided—the primary mixer serves to initially mix an organic feed with an aqueous feed which both enter through a false bottom in the primary mixer and provide the motive force for advancing the resulting dispersion. The one or more auxiliary mixers may be utilized in order to provide adequate residence time for the suspension to affect the mass transfer of a valuable metal from one phase to the other. The suspension then moves to a settling area where a lighter organic phase rises and forms an organic layer, leaving a heavier aqueous layer on the bottom of the settler. The organic fraction may be collected via a fixed organic weir which extends across the entire width of the settler, and then advances to the next stage in the process. The aqueous fraction flows around (e.g., below) the organic weir, and then up and over an adjustable aqueous weir. Fluid flowing over the adjustable aqueous weir may be collected and may either be returned to the primary mixer of the same mixer settler to supplement the aqueous feed or advanced to the next process step. In some instances, a second fixed aqueous weir may be provided downstream of the adjustable aqueous weir, wherein fluid passing over the fixed aqueous weir may be collected and advanced to another stage in the process.